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Subcortical and Spinal Motor control

The ease of access of the motor cortex (both invasively and non-invasively) and its obvious importance in fine finger movements has gravitated research and views regarding motor control.

Despite its undoubted importance there are several other structures, many sub-cortical, that are necessary to bring about efficient and co-ordinated movements.


Reticular Formation

The brainstem is essential for a variety of basal functions such as breathing, heart rate control, swallowing, vomiting and coughing but some of its components also have a role in movement. One such component is the reticular formation (RF), and it has been known for some time to be involved in the postural components of movement1,2,3 , with strong connections to motoneurones of axial muscles 3,5 . However in recent years there has been gathering evidence that it may also have a certain role in voluntary movements of the forelimb3,6,7 and even the hand in the case of primates7,8.

I have recorded from multiple single neurons in the pontomedullary RF during a task involving a slow controlled movement of the index finger 9. Some of the cells in the RF show modulation with these relatively isolated finger movements. The image below shows the muscle activity of an intrinsic hand muscle (First Dorsal Interosseus, 1DI) and the lever position during this task. Underneath is the spike train of an RF cell and its instantaneous firing rate during the same period. There is a strong correlation between the EMG and cell activity.

In some cases there is a spike triggered average (STA) effect from these cells to muscles controlling the digits (example on the right shown below), suggesting a causal relationship from cell to muscle. The STA effect on the right is for the cell shown in the first figure; this implies that the good correlation between cell and EMG activity is more than just correlative and potentially causal.

The precise extent to which the RF has a role in distal movement control is currently not clear. It will not be as distally biased as M1 is but it may instead have a role in the co-ordination of movements involving both distal and proximal musculature.3


Spinal Cord

The spinal cord is often seen as an obedient servant of supra-spinal motor centres, obeying descending instructions faithfully and not dealing with anything more complicated than stretch or withdrawal reflexes. Recordings during behaviour however have demonstrated that the spinal cord is more than that. Spinal interneurons can show preparatory activity preceding movements10; this suggests that spinal cord is likely to be part of the distributed network involved in movement preparation. Does this include bimanual movements?

In a series of acute experiments11, the intracellular responses of arm and hand motoneurones to contralateral cord stimulation were recorded. This was done to assess the commissural connectivity of the primate cervical spinal cord which has not been studied before. Evidence from feline experiments in the hindlimb circuits suggest an important role for commissural system12. However, the forelimb has a role beyond locomotion (and no locomotory role in humans) so it is unclear what the connectivity across the midline is. If there and strong, could it potentially be used for bilateral manipulative movements?

Several of the recorded motoneurones (MN) showed a strong response (two examples with an EPSP are shown below), showing that they receive inputs from the contralateral cord. In the figure below, plot A shows the response of a MN to single shock contralateral intraspinal stimulation (blue arrow). Plot B shows the response of MN to triple shock contralateral intraspinal stimulation (blue arrow). This MN had no response to single shock. Finally, plot C shows the incidence of MN responses to contralateral intraspinal stimulation. MNs were identified as forearm flexor, forearm extensors and intrinsic hand by antidromic identification through appropriate nerve cuffs. In red is the number of responses with monosynaptic latency.


The role (if any) in awake behaviour is unclear, but it implies that spinal cord circuitry could also be involved in bimanual co-ordination.


    References

  • 1: Orlovski GN (1970) Connections of the reticulo-spinal neurons with the "locomotor sections" of the brainstem. Biofizika 15:171-178.
  • 2: Drew T, Dubuc R, Rossignol S. Discharge patterns of reticulospinal and other reticular neurons in chronic, unrestrained cats walking on a treadmill. J Neurophysiol. 1986 Feb;55(2):375-401.
  • 3: Schepens B, Stapley P, Drew T. Neurons in the pontomedullary reticular formation signal posture and movement both as an integrated behavior and independently. J Neurophysiol. 2008 Oct;100(4):2235-53. Epub 2008 Jul 16.
  • 4: Kuypers HG (1981) Anatomy of the descending pathways. In: Handbook of physiology-the nervous system II (Brookhart JM, Mountcastle VB, eds), pp 597-666. Bethesda, MD: American Physiological Society.
  • 5: Peterson BW, Pitts NG, Fukushima K. Reticulospinal connections with limb and axial motoneurons. Exp Brain Res. 1979 Jun 1;36(1):1-20.
  • 6: Buford JA, Davidson AG. Movement-related and preparatory activity in the reticulospinal system of the monkey. Exp Brain Res. 2004 Dec;159(3):284-300. Epub 2004 Jun 25.
  • 7: Davidson AG, Schieber MH, Buford JA. Bilateral spike-triggered average effects in arm and shoulder muscles from the monkey pontomedullary reticular formation. J Neurosci. 2007 Jul 25;27(30):8053-8.
  • 8: Riddle CN, Edgley SA, Baker SN. Direct and indirect connections with upper limb motoneurons from the primate reticulospinal tract. J Neurosci. 2009 Apr 15;29(15):4993-9.
  • 9:Soteropoulos DS, Riddle CN, Williams ER, Baker SN, Cells in the Monkey Ponto-medullary Reticular Formation Modulate Their Activity with Finger Movements, submitted to Journal of Neurophysiology
  • 10: Prut Y, Fetz EE. Primate spinal interneurons show pre-movement instructed delay activity. Nature. 1999 Oct 7;401(6753):590-4.
  • 11:Soteropoulos DS, Edgley SA, Baker SN, Forelimb motoneurone responses to contralateral cord stimulation, manuscript in preparation.
  • 12: Jankowska E. Spinal interneuronal networks in the cat: elementary components. Brain Res Rev. 2008 Jan;57(1):46-55.